The present invention relates to a filter which is used in a selective separation of signals in a particular frequency band in the field of a mobile communication, a satellite communication, a fixed microwave communication and other communication technologies, for example, and in particular, to such a filter which is contained in a metal casing.
Recently, a filter which uses a superconductor is proposed as a filter which is used in the separation of signals in the transmission and reception of a microwave communication, and a variety of constructions are used to construct such a filter including a cavity resonator construction, a microstrip line construction, a coplanar line construction in a flat sheet circuit configuration or the like.
The concept of a coplanar line will be described with reference to FIG. 1. In FIG. 1, formed on a dielectric substrate 1 are a ribbon-like center conductor 2, and a first and a second ground conductor 3a and 3b which are equally spaced from the center conductor 2 on the opposite sides thereof. The three members including the center conductor 2, the first and the second conductor 3a and 3b are formed parallel to and coplanar with each other on the common surface of the dielectric substrate 1. The coplanar line has features that no via-holes are required in forming a one-quarter wavelength resonator, a miniaturization is possible without changing a characteristic impedance and that a greater freedom of design is available. Denoting the width of the center conductor 2 by w and the spacing between the center conductor 2 and each of the first and the second ground conductors 3a and 3b by s, the coplanar line has a characteristic impedance which is determined by the line width w of the center conductor and the spacing d(w+2s) between the first and the second ground conductor 3a and 3b. 
Referring to FIGS. 2A to 2C, a conventional example of the coplanar waveguide filter will be described. This example is what is disclosed in a literature: H. Suzuki, Z. Ma, Y. Kobayashi, K. Satoh, S. Narahashi and T. Nojima, “A low-loss 5 GHz bandpass filter using HTS quarter-wavelength coplanar waveguide resonators”, IEICE Trans. Electron., vol. E-85-C, No. 3, pp714–719, March 2002. In this example, a first to a fourth resonator 5a to 5d are disposed on a line. Each resonator comprises a center conductor 2 having an electrical length equivalent to one-quarter wavelength and a first and a second ground conductor 3a and 3b disposed on the opposite sides of and parallel to the center conductor 2 and spaced therefrom by a spacing s, which are formed on the common surface of a dielectric substrate 1.
A first input/output terminal section 4a of a coplanar line type to which a signal is input is capacitively coupled to the first resonator 5a. In the example shown, one end of a center conductor 24a of the first input/output terminal section 4a and one end of a center conductor 2R1 of the first resonator 5a are disposed in mating relationship with each other in the manner of comb teeth and spaced by a gap g1 in order to strengthen the capacitive coupling, thus forming a first capacitive coupler 6a. The other end of the center conductor 2R1 and one end of a center conductor 2R2 of a second resonator 5b are connected together by shorting line conductors 7a1 and 7a2, which are in turn connected to the first and the second ground conductor 3a and 3b, respectively, thus forming a first inductive coupler 8a between the first and the second resonator 5a and 5b. 
Cuts 20 are formed into the first and the second ground conductor 3a and 3b on each side of the shorting line conductors 7a1 and 7a2, whereby the shorting line conductors 7a are apparently extended, increasing the degree of coupling of the first inductive coupler 8a. A gap g2 is provided between the other end of the center conductor 2R2 of the second resonator 5b and one end of a center conductor 2R3 of a third resonator 5c, whereby the second and the third resonator 5b and 5c are coupled together by a second capacitive coupler 6b. 
The other end of the center conductor 2R3 and one end of a center conductor 2R4 of a fourth resonator 5d are connected together by shorting line conductors 7b1 and 7b2 and connected to the ground connectors 3a and 3b through these shorting line conductors 7b1 and 7b2, whereby the third and the fourth resonator 5c and 5d are coupled together by a second inductive coupler 8b. In the second inductive coupler 8b, also cuts 21 are formed into the ground conductors 3a and 3b 
The fourth resonator 5d and a second input/output terminal section 4b are capacitively coupled. Specifically, the other end of the center conductor 2R4 and a center conductor 24a of the second input/output terminal section 4b are formed in the configuration of meshing comb teeth and disposed in opposing relationship and spaced apart by a gap g3, thus forming a third capacitive coupler 6c which provides a strong coupling therebetween.
In order to reduce a loss caused by an irradiation of electromagnetic power from the filter which defines a coplanar waveguide filter, it is contained in a square tubular metal casing 10 as shown in FIG. 3, for example, allowing the electromagnetic power which is irradiated from the coplanar waveguide filter to be recovered by the filter again. The coplanar waveguide filter 11 is disposed in opposing relationship and parallel to one side plate of the metal casing 10, and the internal space of the metal casing is substantially halved by the coplanar waveguide filter 11. The electromagnetic power which is irradiated from the coplanar waveguide filter 11 is reflected by the internal surface of the metal casing 10 substantially in its entirety and a majority of the irradiated electromagnetic power is recovered by the filter 11, thus alleviating a radiation loss.
In a conventional filter which is confined within a metal casing, the electromagnetic power which is irradiated from the filter contained in the metal casing is reflected by the internal surface of the metal casing, and the majority of the electromagnetic power is recovered by the filter. However, a potion of electromagnetic power which is irradiated from the filter becomes an induced current which follows through the metal on the internal surface of the metal casing 10, presenting a problem of radiation loss. This problem is not limited to a coplanar waveguide filter, but also occurs in a microstrip line filter which is contained within a metal casing.